In the production of minerals, e.g., oil and gas, certain lithological properties of a subterrannean reservoir must be determined. One of the most important of these porperties is the porosity of the reservoir. Porosity of a material is the ratio of the aggregate volume of its void or pore spaces (i.e., pore volume) to its gross bulk volume and, in the case of an oil or gas reservoir, is a measure of the capacity within the reservoir rock which is available for storing oil or gas. Normally, porosity is determined by taking core samples from the reservoir and carrying out well-defined measurement techniques on the samples. In practice, cores are obtained in very few wells because of the expense of coring and analysis and because of the long delay in obtaining results.
Consequently, there is a crucial need for a simple logging method which provides early porosity measurements in intervals of interest. A logging method which can be used in cased wells has added value. It provides a way to detect by-passed pay intervals and to estimate reserves in older wells where porosity data are unavailable. It is this need for porosity measurements by logging methods to which the present invention is directed.